A hybrid vehicle may operate using both hydrocarbon fuel and electric power. A conventional engine may be fueled by the hydrocarbon fuel. An electric motor is powered by a battery, and can create or supplement the engine's power. There are several levels of hybrid vehicles available or in design. Some definitions:
Basic hybrids will be used to refer to the current generation of hybrid vehicles in which the amount of energy stored as liquid fuel is much greater than the energy capacity of the battery so the vehicle is being propelled by the engine most of the time. The vehicle uses the combined power of both the engine and the motor to achieve acceptable performance in acceleration or hill climbing. Performance may suffer if the battery is completely drained.
A basic hybrid may use the engine to operate a generator which charges the battery at times when the full power of the engine is not needed to propel the vehicle. During braking the electric motor can also act as a generator and recover kinetic energy to replenish the battery.
Pure hybrids or serial hybrids refer to more extreme hybrid vehicles that are being designed. In these “pure” or all-electric-drive hybrids, one or more electric motors are the only source of power to the wheels. The only function of the engine is to run a generator to charge the battery. In this type of vehicle it is even more important that there always be charge in the battery since the vehicle cannot move at all without it.
In a pure hybrid the battery pack is typically much larger than in a basic hybrid. This design also has the advantage that the engine and generator can run while the vehicle is parked or stopped. Because most vehicles spend more time parked than moving in this kind of hybrid the engine can be much smaller than the engine in a conventional vehicle of the same weight.
Plug-in hybrid means one in which the driver has the option of plugging the vehicle into an exterior electric power when it is parked so that the battery does not have to be charged by the engine. Typically they have larger batteries than a basic hybrid. Of course if the battery is low and the vehicle is not plugged in the engine will power a generator to charge the battery as in a basic hybrid. Since electricity purchased from a utility is much cheaper than hydrocarbon fuels in terms of cost per unit of energy, it is advantageous to the user to charge from the grid as often as possible and minimize times the engine is charging the battery.
A plug-in hybrid often has a larger battery, so that on local trips the vehicle may be able to run on battery power except when maximal power is needed and thus achieve a higher effective miles per gallon of hydrocarbon fuel. The capacity to plug in is a feature that can be added to the other types mentioned above so a plug-in hybrid will also be either a basic or pure hybrid.
Solar hybrid will be used to refer to newly proposed hybrid vehicles which have solar panels on the body to provide part of the electricity for the electric motor. It is well known that the area available on the top of a typical car is insufficient to provide enough electricity to power the car. In fact, typically the ratio is ⅛ to 1/10 of the area that would be needed to power such a vehicle. On the other hand, a typical car belonging to an individual is parked 90% of the time. Therefore, if the battery is large enough, solar charging could provide a significant portion of the energy used. The currently proposed solar hybrids may also be plug-in hybrids, so if sunlight is unavailable for any reason (weather, parked underground etc.) the battery can be charged from grid power. In addition since it is a hybrid, the battery can always be charged by the engine.
A controller may be formed by one or more processors associated with the vehicle. The controller runs an optimized control algorithm that determines on a moment-to-moment basis when to use either the engine, the motor or both; in what ratio, and also when to charge the battery from the engine. In pure, plug-in and solar hybrids, the controller also makes decisions about how and when to recharge the battery when the vehicle is stopped or parked. The controller may also adjust the transmission and brakes as necessary to maintain optimal efficiency.